“I can confirm that we are also investigating the accused on suspicion of raping a young Irishwoman in the Algarve in 2004,” Brunswick prosecutor Hans Christian Wolters told AFP.
The woman came forward after seeing pictures of the suspect, named only as 43-year-old Christian B., in the media.
“The victim believes she may have recognised her tormentor. As a result, an investigation was opened here at the end of June,” Wolters added.
In a stunning development in the years-long mystery of Madeleine McCann's disappearance, German prosecutors said in a TV appeal in June that they believed Christian B. had kidnapped and murdered the girl.
Madeleine McCann, who was three at the time, went missing from her family's holiday flat in the Portuguese resort Praia da Luz in 2007 where she was sleeping while her parents dined at a nearby restaurant.
Despite a huge international manhunt, no trace of her has been found.
German prosecutors say they have “concrete evidence” that Madeleine is dead, despite British police continuing to treat her disappearance as a missing persons case.
Christian B. has a long history of sex offences, including child abuse and rape.
He is currently serving time in a German prison for drug trafficking.
He has also been convicted in Germany of the 2005 rape of a 72-year-old woman in Praia da Luz, though the sentence is not yet final because of an extradition dispute.
Police in Germany have also linked Christian B. to other unsolved crimes.
Confirming earlier media reports, Wolters said Christian B. was under investigation for the sexual abuse of a 10-year-old girl in Portugal's Algarve region in April 2007, shortly before Madeleine's disappearance in the same area.
The suspect stands accused of masturbating in front of the girl, Wolters said.